Nitrogen-free corrosion inhibitors having a good buffering effect

ABSTRACT

PCT No. PCT/EP95/04844 Sec. 371 Date Jun. 16, 1997 Sec. 102(e) Date Jun. 16, 1997 PCT Filed Dec. 8, 1995 PCT Pub. No. WO96/18757 PCT Pub. Date Jun. 20, 1996Nitrogen-free aqueous anticorrosion composition containing a) a carboxylic acid having 6 to 44 carbon atoms; and b) aromatic hydroxy compounds having a pKa value in the range of 7.0 to 11 for the hydroxy group, in a weight ratio of 1:2 to 20:1, in the form of a concentrate to be diluted with water by a factor of between 20 and 200 and in the form of a ready-to-use aqueous preparation that preferably contains 0.2 to 2 wt % component a) and 0.1 to 0.6 wt % component b) and has a pH value in the range of 7 to 11.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to corrosion control formulations in the form ofan aqueous emulsion or preferably an aqueous solution for temporarilyprotecting metal surfaces, more particularly iron-containing surfaces,against corrosion. To avoid wastewater pollution, they are free fromnitrogen and are distinguished by a good buffering effect so that,despite the possible introduction and/or microbiological production ofacids, their alkaline pH value can be maintained for prolonged periods.

2. Discussion of the Related Art

The protection of metals susceptible to corrosion, for example iron,aluminium, zinc, copper or their alloys, against corrosion is awidespread technical problem. It arises in particular when the metalparts are not, or not yet, covered with a permanentlycorrosion-inhibiting coating, for example in the form of a paint, onaccount of the particular condition in which they are being treated oron account of their particular field of application. Examples of thisare metal parts involved in technical treatment stages such as, forexample, machining or forming or cleaning and assembled metal componentssuch as, for example, heat exchangers or pipelines which come intocontact in service with corrosive aqueous media. In order to prevent orinhibit corrosion during or between the individual treatment steps orduring the intended use, the metal surfaces are contacted with corrosioninhibitors which provide temporary protection against corrosion. It isoften necessary for technical reasons, for example in water-basedcooling or heating circuits, or desirable in the interests of pollutioncontrol, for example during or after cleaning of the metal surfaces withwater, that the metal surfaces be contacted with the corrosioninhibitors in the form of an aqueous phase. Accordingly, a desirableproperty of corrosion inhibitors is that they should be soluble or atleast dispersible in water.

Various inorganic and organic compounds are known as water-soluble orwater-dispersible corrosion inhibitors. Inorganic corrosion inhibitorscan be based, for example, on chromates, nitrites or phosphates which,unfortunately, are more or less toxicologically and ecologically unsafe.Organic corrosion inhibitors are frequently based on carboxylates,amines, amides or on nitrogen-containing heterocyclic compounds. Thereare serious toxicological objections to the use of secondary amines orcompounds capable of releasing secondary amines on account of thepossibility of nitrosamine formation. However, other nitrogen-containingcompounds are toxicologically unsafe or at least problematical onaccount of the risk of water pollution. Although carboxylic acids do nothave any of these disadvantages, they do not show adequate long-termactivity in practice when the pH value of the inhibitor baths is reducedby the introduction of acid or by microbiological processes to such anextent that the carboxylic acids are present not in the salt form, butin the less effective acid form.

EP-B-341 536 describes a water-soluble corrosion inhibitor systemcontaining alkenyl succinic acid, aryl sulfonyl anthranilic acid andalkanolamines. This system does not satisfy the nitrogen-freerequirement. GB-B-1,238,205 describes a water-solublecorrosion-inhibiting combination consisting of gluconate and benzoate orsalicylate salts. On account of the extremely hydrophilic character ofthe salts used, this combination is limited in its effectiveness.EP-A-294 649 teaches the use of partly or completely neutralizedhydroxyaryl fatty acids as corrosion inhibitors. Although they have agood corrosion-inhibiting effect, they are not readily accessible.

The use of carboxylic acids as corrosion inhibitors, for example incooling lubricants, cleaners and corrosion-inhibiting emulsions, iswidespread in the prior art. For example, DE-A42 29 848 describes acooling lubricant emulsion of which the corrosion-inhibiting system isbased on a combination of long-chain fatty acids, short-chain fattyacids, dimer fatty acids and aromatic carboxylic acids, such as forexample benzoic acid or salicylic acid. In this case, the carboxylicacids are neutralized with potassium hydroxide. This system does notcontain a buffer component on the lines of the present invention.

German patent application P 43 23 909 describes two-componentformulations for cleaning and/or passivating metal surfaces, onecomponent containing the active corrosion inhibitors. The activecorrosion inhibitors are selected from

i) at least one carboxylic acid corresponding to general formula (III):

    R--COOY                                                    (III)

where R is a linear or branched, saturated or unsaturated alkyl oralkenyl group containing 5 to 21 carbon atoms or a group correspondingto general formula (IV): ##STR1## where R¹ is a saturated, linear orbranched alkyl group containing 6 to 18 carbon atoms and Y is hydrogen,an alkali metal ion equivalent or an ammonium ion,

ii) substituted benzoic acids,

ii) benzene sulfonamidocarboxylic acids,

iii) aliphatic dicarboxylic acids containing 2 to 36 carbon atoms and

iiii) the salts of the acids mentioned under i) to iii) and mixturesthereof.

This system also does not contain any buffer components for stabilizingthe alkaline pH value in the event of introduction or formation of anacid.

Finally, EP-A-556 087 teaches that monocarboxylic acids containing anodd number of carbon atoms, more particularly heptanoic acid, nonanoicacid and undecanoic acid, are particularly effective corrosioninhibitors.

SUMMARY OF THE INVENTION

The problem addressed by the present invention was to providenitrogen-free water-soluble or water-dispersible corrosion controlformulations in which the active corrosion inhibitor would consist ofcarboxylic acids and which would be safer to use and have a longer lifethrough stabilization against changes in pH.

This problem has been solved by a nitrogen-free water-based corrosioncontrol formulation containing

a) carboxylic acid anions containing 6 to 44 carbon atoms and

b) aromatic hydroxy compounds having a pK_(a) value for the hydroxygroup of 7.0 to11

in a ratio by weight of 1:2 to 20:1.

DETAILED DESCRIPTION OF THE INVENTION

The aromatic hydroxy compound acts as a buffer component which makes anadditional contribution to the corrosion-inhibiting effect. The bufferedcorrosion control system according to the invention is used in the formof an aqueous emulsion or preferably in the form of an aqueous solution,the particularly effective pH range extending from 8.5 to 10. Theinvention is based on the observation that the aromatic hydroxycompounds with a pK_(a) value for the hydroxy group of 7.0 to 11 developa particularly good buffering effect in the above-mentioned pH range ofthe ready-to-use formulation. Aromatic hydroxy compounds with a pK_(a)value for the hydroxy group of 8.5 to 10 are particularly favorable foruse in the preferred pH range of 8.5 to 10. Salicylic acid, which has apK_(a) value for the hydroxy group of 13.4 (N. Konopik, O. Leberl:"Dissoziationskonstanten sehr schwacher Sauren (Dissociation Constantsof Very Weak Acids)", Monatshefte 80 (1949), pages 660 to 662), does notsatisfy this requirement.

It is known that the pK_(a) value is a negative decadic logarithm of theacid constant K_(a) which is generally known as a thermodynamic valueand which is a measure of the completeness of the proton transferreaction from the acid to water and hence of the acid strength. Relevantparticulars can be found in textbooks on general chemistry, cf. forexample H. R. Christen: "Grundlagen der aligemeinen und anorganischenChemie", Verlag Sauerlander, Aarau und Disterweg.Salle, Frankfurt, 4thEdition 1973, pages 353-372. The pK_(a) value for the hydroxy group ofthe aromatic hydroxy compounds according to the invention is based onthe acid/base reaction of the hydroxy group of the aromatic hydroxycompound with water. The aromatic hydroxy compounds to be used inaccordance with the invention may contain carboxylic acid or sulfonicacid groups of which the pK_(a) values are considerably lower and are ofno significance to the buffering effect utilized in accordance with thepresent invention, but can improve the corrosion-inhibiting effect ofthe system as a whole.

The corrosion control formulations according to the invention are usedin the form of an aqueous preparation, i.e. in the form of an aqueousemulsion or preferably in the form of an aqueous solution. Thispreparation preferably contains 0.2 to 2% by weight of component a), 0.1to 0.6% by weight of component b) and, for the rest, water and chemicalsfor establishing an alkaline pH value, preferably alkali metalhydroxides, potassium hydroxide being preferred for reasons ofsolubility in the concentrate. This water-based preparation may containother auxiliaries depending on the application envisaged. The pH valueof the waterbased preparation is in the range from 7 to 11 andpreferably in the range from 8.5 to 10, for example 9.3.

Component a) is preferably selected from monobasic or polybasic,preferably monobasic, saturated or mono- or polyunsaturated linear orbranched carboxylic acids containing 6 to 22 carbon atoms and/orunsaturated polybasic, preferably dibasic, carboxylic acids containing36 to 44 carbon atoms. Examples of carboxylic acids containing 6 to 22carbon atoms are the unbranched saturated carboxylic acids hexanoicacid, octanoic acid and decanoic acid and, in particular, themonocarboxylic acids containing an odd number of carbon atoms which arepreferably used under the teaching of EP-A-556 087, more particularlyheptanoic acid, nonanoic acid and undecanoic acid. Particular technicaladvantages are also afforded by branched saturated carboxylic acids,more particularly 2ethyl hexanoic acid, 3,5,5-trimethyl hexanoic acidand 2,2-dimethyl octanoic acid. One example of a preferred unsaturatedshort-chain carboxylic acid is 1 ,4-hexadienoic acid (sorbic acid). Inaddition, saturated or unsaturated fatty acids containing 12 to 22carbon atoms in the molecule are generally usable as are the technicalmixtures thereof which are obtained, for example, in the hydrolysis ofnatural fats and oils or which may even be synthesized. Examples includeoleic acid and the technical mixtures of various fatty acids known astall oil fatty acid which may be obtained from tall oil and whichconsists mainly of linoleic acid and conjugated C₁₈ fatty acids, oleicacid and 5,9,12-octadecatrienoic acid. In addition, alkylphenyl-substituted unsaturated carboxylic acids of the following type:

    R'--C.sub.6 H.sub.4 --C(O)--CH═CH--COOH

or

    R'--C.sub.6 H.sub.4 --CH═CH--COOH

where R' is a linear or branched alkyl group containing 8 to 14 carbonatoms, may also be used.

Unsaturated polybasic carboxylic acids containing 36 to 44 carbon atoms,of which the dibasic representatives are preferably used, are also knownas "dimer fatty acids". They may be obtained on an industrial scale by--generally acid-catalyzed--dimerization of suitable unsaturated fattyacids containing 18 to 22 carbon atoms. The reaction products aregenerally mixtures of acids with different degrees of oligomerizationtogether with unreacted or isomerized starting materials. Such productsare commercially obtainable, for example from Unichema under the productgroup name of Pripol® or from Henkel KGaA under the product group nameof Empol®.

Besides the carboxylic acid components a), of which thecorrosion-inhibiting effect is already known in the prior art, thearomatic hydroxy compound b) acting as buffer is also particularlyimportant in stabilizing the corrosion-control formulation against theintroduction or formation of acid and, hence, in keeping it usable for alonger period than non-buffered systems in the absence of furthercorrective measures. The aromatic hydroxy compounds selected have theadvantage over other possible buffer systems for the pH range from 7 to11 that they support the corrosion-inhibiting effect of the carboxylicacid components. Component b) is preferably selected from mono-, di- ortrinuclear aromatic hydroxy compounds corresponding to general formula(I):

    HO--Arom--X                                                (I)

where X is selected from the substituents COOH, SO₃ H or SO₂ --C₆ H₄--OH and "Arom" represents a mono-, di- or trinuclear carbocyclicaromatic 6-membered ring system of the benzene, naphthalene, anthraceneor phenanthrene type which may contain further substituents X, OH and/oralkyl, hydroxyalkyl and/or hydroxyalkyl ether groups containing 1 toabout 4 carbon atoms. Hydroxyalkyl ether groups such as, for example,ethylene glycol ether or propylene glycol ether groups can be obtained,for example, by ethoxylation or propoxylation of phenols.

Component b) is preferably selected from mononuclear aromatic hydroxycompounds which may be represented by general formula (II): ##STR2## inwhich X is selected from the substituents COOH, SO₃ H or SO₂ --C₆ H₄--OH and R¹, R², R³ and R⁴ independently of one another represent agroup X, H, OH or an alkyl, hydroxyalkyl or hydroxyalkyl ether groupcontaining 1 to 4 carbon atoms.

Preferred compounds of general formula (II) are those which contain onlyone substituent X, preferably in the para position to the hydroxylgroup, and in which the substituents R¹, R², R³ and R⁴ are preferablyhydrogen. The substituent X is preferably one of the groups COOH, SO₃ Hor SO₂ --C₆ H₄ --OH. Particularly suitable examples of such compoundsare 4-hydroxybenzoic acid, 4-phenol sulfonic acid and4,4'-dihydroxydiphenyl sulfone (bisphenol S).

Compounds containing a carboxylic acid or sulfonic acid function willlargely be present in the form of their carboxylate or sulfonate anionsat the pH value of the in-use solution of 7 to 11. Since hydroxycompounds of which the pK_(a) value for the hydroxy group is 7.0 to 11are used in accordance with the invention, the hydroxy groups can beexpected to be partly deprotonated in the pH range selected. Particularsof the determination of the pK_(a) values of the only weakly acidichydroxy groups can be found in the article by Konopik cited earlier on.Further information on pK_(a) measurements can be found in: B. Jones, J.C. Speakman: "Thermodynamic Dissociation Constants of Hydroxy- andAlkoxy-benzoic Acids", J. Chem. Soc. 1944, pages 19 to 20. According tothis article, m- and p-hydroxybenzoic acid have pK_(a) values of 9.94and 9,39, respectively. The pK_(a) value of 4-phenol sulfonic acid forthe phenolic hydroxy group was found to be 8.9 while that of bisphenol Swas found to be 9.5. By contrast, the dissociation constant of salicylicacid in regard to the OH group is so low that it could not be determinedby the method selected here of potentiometric titration using a glasselectrode.

For its required long-term stability, the ready-to-use aqueouscorrosion-inhibiting solution or dispersion should preferably have abuffering capacity of 1 to 8. The buffering capacity is defined as theconsumption of 0.1 normal hydrochloric acid, as measured in ml, requiredto lower the pH value of 50 g of the corrosion-control formulation by1.3 units, for example to titrate 50 g of the corrosion-controlformulation from a pH value of 9.3 to a pH value of 8. Where componentsa) are used, buffering capacities of this order can be obtained bycombining the components a) with components b) in accordance with thepresent invention.

Buffering capacities of a similar order could also be obtained bycombining components a) with other buffers, for example with suitableamines, borates or phosphates. However, amines should not be used forthe reasons mentioned at the beginning. The teaching according to theinvention also eliminates the need to use the other ecologically unsafebuffer systems, such as borates or phosphates.

The concentrations mentioned above for the corrosion-controlformulations suitable for use in accordance with the invention apply tothe ready-to-use aqueous preparations. In principle, they may beprepared in situ by dissolving or dispersing the individual componentsin water in the concentration ranges mentioned. However, it is standardpractice in the technical field in question to market active-substancecombinations such as these in the form of concentrates which alreadycontain all components in the necessary quantity ratios and from whichthe ready-to-use treatment baths can be prepared in situ by dilutionwith water. Accordingly, the present invention also relates towater-based active-substance concentrates which contain components a)and b) in the quantities shown in claim 1. The active-substanceconcentrations in these concentrates are preferably adjusted in such away that an aqueous corrosion control formulation having the propertiesdescribed above is obtained by diluting the concentrate with water by afactor of about 20 to about 200.

EXAMPLE S

According to the Table, active-substance concentrates were prepared fromthe inhibitor component a) and the buffer component b) by dissolving ordispersing the individual components in deionized water in theconcentrations shown in the Table and adding KOH in such a quantity thata solution diluted by a factor of 50 with deionized water had a pH valueof 9.3.

The buffering capacity was determined by titrating 50 g of theconcentrate solution diluted in a ratio of 1:50 with 0.1 normalhydrochloric acid from pH 9.3 to pH 8. The consumption of hydrochloricacid in ml required for this purpose is the buffering capacity.

The corrosion control effect was tested in a plate conditioning test. Tothis end, ST 1405 steel plates measuring 5 cm×10 cm were brushed with anaqueous surfactant solution, rinsed with water and alcohol and dried.The plates were then immersed in the concentrate solutions according tothe Table diluted by a factor of 50 with deionized water, allowed todrain and then placed in a conditioning chamber at 22° C./76% relativeair humidity. All the Examples produced the necessary corrosion control(less than 30% corrosion after a test duration of 40 days). Theformulation of Example 10 was also tested in more heavily diluted form.The necessary corrosion-inhibiting effect was also achieved when theconcentrate was diluted by a factor of 75.

                  TABLE                                                           ______________________________________                                        Active-substance Concentrates, Concentrations in % by weight                  in Water/KOH                                                                                                          Buffering                                                                Conc.                                                                              capacity                              No.     Inhibitor a)                                                                              Conc.  Buffer b)                                                                             (%)  (ml)                                  ______________________________________                                        Comp. 1 Heptanoic acid                                                                            20     --           0.06                                  Comp. 2 Sorbic acid 20     --           0.03                                  Comp. 3 Heptanoic acid                                                                            20     Salicylic                                                                             5    <0.1                                                             acid                                               Example 1                                                                             tall oil fatty acid                                                                       20     4-Hydroxy-                                                                            5    1.6                                                              benzoic acid                                       Example 2                                                                             Oleic acid  20     4-Hydroxy-                                                                            5    2.6                                                              benzoic acid                                       Example 3                                                                             Dimer fatty acid                                                                          20     4-Hydroxy-                                                                            5    1.6                                           (Empol ® 1022, benzoic acid                                               Henkel KGaA)                                                          Example 3                                                                             Heptanoic acid                                                                            20     4-Hydroxy-                                                                            5    2.0                                                              benzoic acid                                       Example 4                                                                             Sorbic acid 20     4-Hydroxy-                                                                            5    1.8                                                              benzoic acid                                       Example 5                                                                             Sorbic acid 20     4-hydroxy-                                                                            10   5.0                                                              benzoic acid                                       Example 6                                                                             Sorbic acid 20     4-hydroxy-                                                                            20   6.0                                                              benzoic acid                                       Example 7                                                                             Sorbic acid 20     Bisphenol S                                                                           5    1.6                                   Example 8                                                                             Heptanoic acid                                                                            20     4-Phenol-                                                                             5    1.6                                                              sulfonic acid                                      Example 9                                                                             Heptanoic acid +                                                                          10     4-Hydroxy-                                                                            5    2.0                                           sorbic acid 10     benzene                                                                       sulfonic acid                                       Example 10                                                                           3.5.5-trimethyl                                                                           10     4-Hydroxy-                                                                            10   5.2                                           hexanoic acid +    benzene                                                    sorbic acid 10     sulfonic acid                                      ______________________________________                                    

We claim:
 1. An aqueous corrosion control formulation comprising:a. acarboxylic acid having 6 to 44 carbon atoms; and b. an aromatic hydroxycompound having a pK_(a) of 7.0 to 11, wherein a. and b. are present ina weight ratio of 1:2 to 20:1, wherein said formulation is free ofnitrogen, and wherein said formulation has a buffering capacity of 1 to8.
 2. A corrosion control formulation according to claim 1, comprising0.2% to 2% by weight carboxylic acid, 0.1% to 0.6% by weight aromatichydroxy compound, and an amount of a base sufficient to provide saidformulation with an alkaline pH.
 3. A corrosion control formulationaccording to claim 2, wherein said base is an alkali metal hydroxide. 4.A corrosion control formulation according to claim 3, wherein said baseis potassium hydroxide.
 5. A corrosion control formulation according toclaim 2, wherein said pH has a value of 7 to
 11. 6. A corrosion controlformulation according to claim 5, wherein said pH has a value of 8.5 to10.
 7. A corrosion control formulation according to claim 1, whereinsaid carboxylic acid is:a. a mono- or polybasic, mono- orpolyunsaturated or saturated, linear or branched carboxylic acid having6 to 22 carbon atoms; or b. an unsaturated polybasic carboxylic acidhaving 36 to 44 carbon atoms.
 8. A corrosion control formulation asclaimed in claim 1, wherein said aromatic hydroxy compound is a compoundof the formula (I)

    HO--Arom--X                                                (I)

wherein X is COOH, SO₃ H, or SO₂ --C₆ H₄ --OH and Arom is a mono-, di-,or trinuclear carbocyclic aromatic 6-membered ring that can be furthersubstituted by X, OH, C₁ -C₄ -alkyl, C₁ -C₄ -hydroxyalkyl, or C₁ -C₄-hydroxyalkyl ether.
 9. A corrosion control formulation according toclaim 8, wherein said aromatic hydroxy compound is a compound of theformula (II) ##STR3## wherein X is COOH, SO₃ H, or SO₂ --C₆ H₄ --OH andR¹, R², R³, and R⁴ are independently X, H, OH, C₁ -C₄ -alkyl, C₁ -C₄-hydroxyalkyl, or C₁ -C₄ -hydroxyalkyl ether.
 10. A corrosion controlformulation according to claim 9, wherein R¹, R², R³, and R⁴ are H. 11.A corrosion control formulation according to claim 1, wherein saidcarboxylic acid is selected from the group consisting of hexanoic acid,octanoic acid, decanoic acid, nonanoic acid, undecanoic acid,2-ethylhexanoic acid, 2,2-dimethyloctanoic acid, linoleic acid,conjugated C18 fatty acids, 5,9,12-octadecatrienoic acid, tall oil fattyacid, oleic acid, dimer fatty acid, heptanoic acid, sorbic acid, and3,5,5-trimethylhexanoic acid and said aromatic hydroxy compound isselected from the group consisting of 4-hydroxybenzoic acid, bisphenolS, 4-phenolsulfonic acid, and 4-hydroxybenzene sulfonic acid.